1. Field of the Invention
The present invention relates to radio frequency communication receivers, systems and methods employing ultra wide bandwidth (UWB) signaling techniques. More particularly, the present invention relates to the systems and methods configured to control in a receiver when to acquire the UWB signal and when to track the incoming UWB signal to maintain quality of service.
2. Description of the Background
In wireless communication systems, a transmitter takes data and sends the data to an amplifier and antenna, which converts the data from electrical signals into electromagnetic radiation. This electromagnetic radiation propagates through the air and is converted into an electrical current by an antenna coupled to a receiver. These currents (or voltage) are then amplified before being sent to a converter to convert the electrical signals into digital samples and subsequently processed to extract the source information from the signal.
In order to maintain a particular quality of service at the receiver, the receiver “locks” on to the incoming signal. Thus, the receiver monitors the signal strength of the incoming signal, and employs a device to determine when the receiver should be placed in a signal tracking mode of operation or a signal acquire mode of operation. More detailed descriptions of receiver synchronization are found in Chapter 8 of “Digital Communications” B. Sklar, Prentice Hall, 1988, the entire contents of which are incorporated by reference herein.
Some radios have some type of mode controller incorporated into the receiver. The mode controller monitors the received incoming signal and determines whether the signal to noise ratio (SNR) is sufficient to maintain an acceptable quality of service. If the mode controller determines that the SNR is not sufficient, the receiver is forced out of a tracking state and into an acquisition state.
Some radios use a RSSI (received signal strength indicator) to determine what mode, i.e., tracking or acquisition, the mode controller should be in. The RSSI measures purely incoming signal strength. However, a problem with these type of controllers is that when the noise power increases significantly, the signal strength still shows acceptability when, in fact, the quality of the signal is noisy and unacceptable.
Other radios use two RSSIs—one to measure signal power and the other to measure noise power. The noise power is measured in an out-of-band region of the spectrum presumably unoccupied by any signals. Assuming the noise is the same in the out-of band region as in the in-band region, this measure presumably indicating an accurate noise power for the in-band region. Consequently, the out-of-band noise power is presumed to be the same as the in-band noise power, i.e. noise coupled with the incoming signal. Neither presumption may be correct. The presumed unoccupied region may contain some signal, which would affect the estimate of the assumed noise power, and the out-of-band noise power may not be the same as the in-band noise power. These radios estimate SNR from the in-band signal measure and out-of-band noise measure. The underlying presumption that noise changes little over relatively small frequency ranges empowers such techniques for narrowband systems. Out-of-band noise for UWB systems holds no significance. Hence, a truer estimate of SNR is desired.
The present inventors recognize that in order to get a true indication of radio performance, both signal and noise power should be measured and both measurements should be taken in-band, especially for UWB systems. The true indication of radio performance allows the mode controller to accurately switch between the acquisition and tracking states of the radio, preventing erroneous acquisitions, which adversely affect system throughput because the receiver spends time trying to acquire a signal when it should be receiving data at an acceptable bit error rate (BER).
Such erroneous transitions to the acquisition mode arise in systems where the incoming signal is prone to burst error or intermittent signal loss, for example. The bursty nature of the incoming signal is particularly true for a UWB channel. In these bursty communication channels, the receiver can frequently forced out of the tracking state, due to a short outage, no longer receiving the signal. The radio attempts to reacquire the signal in order to get an acceptable SNR even though the reception outage time is relatively short. These frequent reception interruptions while the radio attempt reacquisition adversely affect the system's effective throughput.
The challenge is to effectively determine when a receiver should transition between a tracking state and an acquisition state in a way that minimizes degradation of quality of service (e.g., acceptable BER at a certain throughput).